If I were a member...
SCA participants are just plain folks who enjoy doing something more with their weekends. People from all walks of life join the SCA – students, teachers, historians, writers, secretaries, law enforcement personnel, chemists, and insurance agents. Many SCA participants are involved in high tech fields – computers, aerospace, high energy physics, etc. People who spend all week with highly complex, modern technology can find it relaxing to spend their leisure time using simpler technologies in a less modern setting.
Members of the SCA study and take part in a variety of activities, including combat, archery, equestrian activities, costuming, cooking, metalwork, woodworking, music, dance, calligraphy, fiber arts, and much more. If it was done in the Middle Ages or Renaissance, odds are you'll find someone in the SCA interested in recreating it.
What makes the SCA different from a Humanities 101 class is the active participation in the learning process. To learn about the clothing of the period, you research it, then sew and wear it yourself (though of course there are merchants to purchase clothing from too). To learn about combat, you put on armor (which you may have built yourself) and learn how to defeat your opponent. To learn brewing, you make (and sample!) your own wines, meads and beers.
You will frequently hear SCA participants describe the SCA as recreating the Middle Ages "as they ought to have been." In some ways this is true – we choose to use indoor plumbing, heated halls, and sewing machines. In the dead of winter we have more to eat than King's venison, salt pork and dried tubers. However, a better description is that we selectively recreate the culture, choosing elements of the culture that interest and attract us.